Re-Polygonize a tessellated / triangulated mesh?
Assuming you have no way to access the original (non-triangulated mesh), there's no way to get the exact polygons back, which may have existed at some point in some other application. But you can try several things in Blender, to get some polygons from any triangulated or tessellated model.


You can try the Limited Dissolve operator (X), which removes less important geometry using an angle to determine the importance (a subdivided plane will turn into a single quad using an minimal angle of 0.001°, since it's flat).There's also a modifier, that acts the same, but non-destructive: Decimate set to Planar.
Less automated, but potentially better results are achievable with Dissolve Edges. Switch to edge select mode, select the edges you wanna get rid of and execute the operator (X).
You can also select multiple triangles and try to join them using Make Edge/Face (F).